1. Field of the Invention
This invention relates to methods and apparatus to prepare good quality precursor films that are converted into solar cell absorbers.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu. In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax(SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohimic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) or a grid pattern may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1. It should be noted that although the chemical formula for a CIGS(S) layer is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
One technique employed for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two ingredients or elements or components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 or CIS film growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor structure is reacted with Se at elevated temperature to form CIS. If the reaction atmosphere also contains sulfur, then a CuIn(S,Se)2 or CIS(S) layer can be grown. Addition of Ga in the precursor structure, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 or CIGS(S) absorber.
Sputtering and evaporation techniques have been used in prior art approaches to deposit the layers containing the Group IB and Group IIIA components of the precursor stacks. In the case of CuInSe2 growth, for example, Cu and In layers were sequentially sputter-deposited on a substrate and then the stacked film was heated in the presence of gas containing Se at elevated temperature for times typically longer than about 30 minutes, as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy layer and an In layer to form a Cu—Ga/In stack on a metallic back electrode layer and then reacting this precursor stack film with one of Se and S to form the absorber layer. Electron beam evaporated In/Cu/Ga stacks have also been prepared and then reacted with H2Se to form CIGS (see, for example, B. Basol et al., J. Vacuum Science and Technology A, 14 (1996) 2251).
One prior art method described in U.S. Pat. No. 4,581,108 utilizes a low cost electrodeposition approach for metallic precursor preparation. In this method a Cu layer is first electrodeposited on a substrate. This is then followed by electrodeposition of an In layer and heating of the deposited Cu/In stack in a reactive atmosphere containing Se to form CIS. Various other researchers have reported In electroplating approaches for the purpose of obtaining In-containing precursor structures later to be converted into CIS absorber films through reaction with Se (see for example, Lokhande and Hodes, Solar Cells, 21 (1987) 215; Fritz and Chatziagorastou, Thin Solid Films, 247 (1994) 129; Kim et al, Proceedings of the 1st World Conf. on Photovoltaic Energy Conversion, 1994, p. 202; Calixto and Sebastian, J. Materials Science, 33 (1998) 339; Abedin et al., Electrochemica Acta, 52 (2007) 2746, and, Valderrama et al., Electrochemica Acta, 53 (2008) 3714).
In a thin film solar cell employing a Group IBIIIAVIA compound absorber such as CIS or CIGS, the solar cell efficiency is a strong function of the molar ratio of the IB element(s) to IIIA element(s), i.e. the IB/IIIA molar ratio. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the solar cell efficiency and other properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at or below 1.0. For ratios higher than 1.0, a low resistance copper selenide phase, which may introduce electrical shorts within the solar cells may form. Increasing the Ga/(Ga+In) molar ratio, on the other hand, widens the optical bandgap of the absorber layer, resulting in increased open circuit voltage and decreased short circuit current. A CIGS material with a Ga/(Ga+In) molar ratio higher than about 0.3 is electronically poor. It is for this reason that the sunlight-to-electricity conversion efficiency of a CIGS type solar cell first increases as the Ga/(Ga+In) molar ratio in the absorber is increased from 0 to 0.3, and then the efficiency starts to decrease as the molar ratio is further increased towards 1.
In light of the above discussion, it should be appreciated that if any layer in a CIGS(S) precursor stack has non-uniform thickness, such non-uniformity produces micro-scale compositional non-uniformities. If, for example, the micro-structure of an In film or a In—Ga film electroplated on a planar Cu or Cu—Ga alloy layer is rough and includes protrusions and valleys or discontinuities, the localized micro-scale Ga/(In+Ga) ratio at the protrusions would be lower than the Ga/(In+Ga) ratio at the valleys. Furthermore, the Cu/(In+Ga) molar ratio would be different at these two locations. As will be described next, this kind of micro-scale non-uniformity would yield a CIGS(S) absorber with non-uniform electrical and optical properties after reaction of the precursor stack with Se and/or S.
Low melting Group IIIA materials such as In and Ga have high surface tension and they often grow in the form of islands or droplets when deposited on a substrate surface in thin film form. This behavior has been observed in prior work carried out on electroplated In films (see for example, Chen et al., Solar Cells, 30 (1991) 451; Kim et al, Proceedings of the 1st World Conf. on Photovoltaic Energy Conversion, 1994, p. 202; Calixto and Sebastian, J. Materials Science, 33 (1998) 339; Abedin et al., Electrochemica Acta, 52 (2007) 2746, and, Valderrama et al., Electrochemica Acta, 53 (2008) 3714), electroplated In—Ga alloy films (see for example Zank et al., Thin Solid Films, 286 (1996) 259) as well as in e-beam evaporated In films (Chen et al, Solar Cells, 30 (1991) 451). As stated before, lack of micro-scale planarity in In and/or Ga-rich layers presents problems for application of such non-uniform layers to thin film solar cell manufacturing.
FIGS. 2A-2B schematically show a prior art structure in perspective and side views, respectively. The structure includes a typical prior art In layer 37, with sub-micron thickness which may be electrodeposited on a surface 36 of an under-layer 33. The under-layer 33 is formed over a base 30 having a substrate 31 and a contact layer 32. The under-layer 33 may, for example, include Cu and Ga and be formed on the contact layer 32. As can be seen from FIGS. 2A and 2B, the sub-micron thick In layer 37 is discontinuous and it includes islands 34 of In, separated by valleys 35 through which the surface 36 of the under-layer 33 is exposed. The width of the islands may be in the range of 500-5000 nm. Although the top surface of the islands in FIG. 2B is shown as relatively planar, in practice the heights of the individual islands may be different from each other and their top surfaces may be irregular instead of flat (see for example FIG. 4A). In any case, if the structure of FIGS. 2A and 2B is reacted with a Group VIA material such as Se, a CIGS solar cell absorber 40 may be formed on the base 30 as shown in FIG. 3. The CIGS solar cell absorber 40 has compositional non-uniformities caused by the morphological non-uniformity of the sub-micron thick In layer 37. Accordingly, the CIGS solar cell absorber 40 has a first region 41 and a second region 42. The first region 41 corresponds to the islands 34 of In of the structure of FIG. 2A, and is an In-rich, Ga-poor region. The second region 42 corresponds to the valleys 35 of the structure of FIG. 2A, and is an In-poor, Ga-rich region. Furthermore, the Cu(In+Ga) molar ratio in the first region 41 is lower than the Cu(In+Ga) molar ratio in the second region 42. It should be appreciated that when a solar cell is fabricated on the CIGS solar cell absorber 40, the efficiency of the solar cell would be determined by both the first region 41 and the second region 42. The solar cell would act like two separate solar cells, one made on the first region 41 and the other made on the second region 42 and then interconnected in parallel. Since the Ga/(Ga+In) as well as the Cu/(In+Ga) molar ratios in the two regions are widely different the quality of the separate solar cells on these regions would also be different. The quality of the overall solar cell would then suffer from the poor I-V characteristics of the separate solar cells formed on either one of the first and second regions.
As can be seen from the foregoing discussion there is a need to develop approaches that provide substantially planar precursor structures that can be converted into compositionally uniform semiconductor films. Such films can be used in electronic applications such as in processing thin film CIGS type solar cells.